Welding head

ABSTRACT

A welding head comprises a welding element ( 51 ) for welding a lid ( 1 ) to an opening device of a container ( 10 ). The welding head ( 43 ) further comprises a 5 compensating device ( 62 ) operable for compensating a misalignment between the welding element ( 51 ) and the lid ( 1 ), thereby centering the welding element ( 51 ) relative to the lid ( 1 ).

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This is a National Phase of International Application No.PCT/EP2016/072124, filed Sep. 19, 2016, which claims the benefit ofEuropean Application No. 15188370.9, filed Oct. 5, 2015. The entirecontents of the above-referenced applications are expressly incorporatedherein by reference.

TECHNICAL FIELD

The invention relates to a welding head for welding a lid onto acontainer, particularly a sealed container for packaging pourable foodproducts. The welding head according to the invention is especiallysuitable for welding a lid onto a neck that has been moulded on a sheetpackaging material, the latter being in turn adapted to be folded,filled with a pourable food product and sealed to form a container.

BACKGROUND OF INVENTION

As known, many pourable food products, such as fruit juice, UHT(ultra-high-temperature treated) milk, wine, tomato sauce, etc., aresold in containers made of sterilized sheet packaging material.

A typical example of this type of containers is theparallelepiped-shaped container for liquid or pourable food productsknown as Tetra Brik Aseptic (registered trademark), which is made byfolding and sealing laminated strip packaging material.

The packaging material has a multilayer structure substantiallycomprising a base layer for stiffness and strength, which may include alayer of fibrous material, e.g. paper, or mineral-filled polypropylenematerial, and a number of lamination layers of heat-sealable plasticmaterial, e.g. polyethylene films, covering both sides of the baselayer.

In the case of aseptic containers for long-storage products, such as UHTmilk, the packaging material also comprises a layer of gas-barriermaterial, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, whichis superimposed on a layer of heat-sealable plastic material, and is inturn covered with another layer of heat-sealable plastic materialforming the inner face of the container eventually contacting the foodproduct.

Containers of this sort are normally produced on fully automaticpackaging machines, which are fed with a web of packaging material thatis sterilized on the packaging machine, e.g. by applying a chemicalsterilizing agent, such as a hydrogen peroxide solution, which, oncesterilization is completed, is removed from the surfaces of thepackaging material, e.g. evaporated by heating. The web of packagingmaterial so sterilized is then maintained in a closed, sterileenvironment, and is folded and sealed longitudinally to form a verticaltube.

The tube is filled with a sterilized or sterile-processed food product,and is sealed and subsequently cut along equally spaced cross sectionsto form pillow packs, which are then folded mechanically to formrespective finished, e.g. substantially parallelepiped-shaped,containers.

Alternatively, the packaging material may be cut into blanks, which areformed into containers on forming spindles, and the containers arefilled with the food product and sealed. One example of this type ofcontainer is the so-called “gable-top” container known by the trade nameTetra Rex™.

To open the containers described above, various solutions have beenproposed, including reclosable opening devices made of plastic materialand substantially comprising a pouring spout, defining a through pouringopening and fitted to a hole in a wall of the container.

When producing the opening device, the opening of the pouring spout issealed by a closing element connected integrally to the pouring spoutand detachable from it along a normally circular tear line. The closingelement extends at the same level as the packaging material, so as toseal the hole in the wall of the container. On the side facing the lid,the closing element has an integral projecting pull ring, the free endof which is pulled by the user to detach the closing element from thepouring spout along the tear line and so open the pouring opening. Morespecifically, the pull ring extends inside, and at a predetermineddistance from, the pouring spout.

It is also possible to fix the closing element of the opening devicedirectly over a prelaminated hole in the packaging material, i.e. a holeformed in the base layer only and covered by the other laminationlayers, including the layer of gas-barrier material.

In both cases, a removable, e.g. screw or hinged, lid, is subsequentlyfitted to the pouring spout in order to outwardly close the latter.

The opening devices disclosed above have excellent sealing properties.However, they have the drawback that, the first time a user opens acontainer closed by an opening device of the kind disclosed above, twosteps must be performed before being capable of pouring the content ofthe container. In a first step, the user removes the lid removablyfitted to the pouring spout.

In a second step, the user detaches the closing element from the pouringspout along the tear line, by acting on the pull ring.

In order to provide a container that can be opened in one step only, asolution has been proposed, in which the closing element of the openingdevice is formed in one piece with a protruding portion extending insidethe pouring spout and welded to the lid. An end wall of the lid isprovided with a disk-shaped welding promoting element that is welded tothe protruding portion so that, when removing the lid from the pouringspout, the protruding portion and the closing element remain attached tothe lid. Thus, the first time a user opens the container, the lid andthe closing element can be removed from the pouring spout in a singleoperation. A container provided with a lid of the kind disclosed abovecan be called a “one-step-opening container”. In order to weld the lidto the protruding portion integral with the closing element, it ispossible to use an electric induction generating element comprising acoil that, when activated, induces an electric current in a conductivelayer of the welding promoting element. Localized heat is consequentlygenerated in a heat-sealable layer of the welding promoting element,which causes the end wall of the lid—including the welding promotingelement—to become attached to the protruding portion.

To ensure that the end wall of the lid is correctly welded to theprotruding portion, the coil of the electric induction generatingelement needs to be centered relative to the end wall of the lid.

This issue is particularly critical, also in view of the dimensions,shape and position of the neck and the lid, which may vary withinrelatively large tolerances. In particular, the dimensions and shape ofthe container may vary to a certain extent due to the flexible nature ofthe material forming the container, namely a sheet packaging material.The dimensions and shape of the neck may also vary within a relativelylarge range, because the neck is moulded directly on the sheet packagingmaterial forming the container, by using a polymeric material having alimited stiffness. For these reasons, the position of the neck relativeto the corresponding container may vary from a container to another.

Also the lid, which is made of a relatively flexible material such asplastics, may have a shape and dimensions that are not exactlyrepeatable between different lids.

It may therefore happen that an electric induction generating element,which is perfectly centered relative to a lid applied onto a containerneck, is in a non-centered position relative to a subsequent lid appliedonto another container neck.

This situation is worsened because the containers at issue, which arenormally sealed containers for packaging pourable food products, areproduced by machines that operate at significantly high output rates.Hence, the available time for welding a lid to a protruding portion of aneck is limited.

Consequently, it may happen that some lids are not perfectly welded tothe corresponding containers.

DISCLOSURE OF INVENTION

An object of the invention is to provide a welding head that is capableof effectively and reliably welding a lid to an opening device of acontainer.

Another object is to provide a welding head for welding lids ontoopening devices of respective containers, which ensures a good weldingquality even if the containers interact with the welding head at aposition that may vary between different containers.

A further object is to provide a welding head that can be used to weldlids onto respective opening devices of container, which guarantees goodperformances in spite of the containers, and/or the opening devices,and/or the lids, having shapes and/or dimensions that differ from oneanother.

Another object is to provide a welding head that can be used to weld alid to an opening device of a container, which ensures a good weldingquality even at high production rates.

According to the invention, there is provided a welding head comprisinga welding element for welding a lid to an opening device of a container,characterized in that the welding head further comprises a compensatingdevice operable for compensating a misalignment between the weldingelement and the lid, thereby centering the welding element relative tothe lid.

The compensating device allows the welding element to be centeredrelative to subsequently processed lids, even if the lids are inpositions that differ from the respective theoretical positions. A goodwelding quality is thus ensured.

The misalignments between the lid and the welding element, which can becompensated by the compensating device, may in particular be due to avariability in dimensions and/or shape of the neck and/or of the lid.Owing to the compensating device, the lid may be successfully welded tothe opening device, irrespective of the relatively large toleranceswithin which the dimensions or shape of the container, neck or lid arevariable.

Furthermore, by centering the welding element relative to the lid, thewelding time necessary for welding the lid to the opening device may beminimized, which makes the welding head according to the inventioncapable of operating even at high production rates.

In an embodiment, the welding element comprises an electric inductiongenerating element for generating an electric current capable ofheat-sealing the lid to the opening device.

The compensating device allows a coil included in the electric inductiongenerating element to be centered relative to subsequently processedlids. The electric induction field generated by the coil, which islocalized in a relatively narrow zone, may thus be centered relative tothe lid, thereby ensuring a good welding quality.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and carried out with referenceto the enclosed drawings, which show an exemplifying and non-limitingembodiment thereof, in which:

FIG. 1 is a perspective view of a lid intended to be applied onto a neckof a container;

FIG. 2 is an enlarged cross-section showing the lid of FIG. 1, screwedonto a corresponding neck;

FIG. 3 is a perspective view showing the lid and neck of FIG. 2, whenthe lid has been removed from the respective container by a user;

FIG. 4 is a perspective view showing an apparatus for applying lids ontocorresponding necks of containers;

FIG. 5 is an enlarged perspective view, showing two applying heads andtwo welding heads of the apparatus of FIG. 1, in a workingconfiguration;

FIG. 6 is a view like FIG. 5, showing the applying heads and the weldingheads in a different working configuration;

FIG. 7 is an enlarged front view, showing a detail of an applying headin the working configuration of FIG. 5;

FIG. 8 is an enlarged plan bottom view, showing two gripping elements ofthe applying head of FIG. 7 coupled to a lid;

FIG. 9 is a view like FIG. 5, showing the applying heads and the weldingheads in another working configuration;

FIG. 10 is an enlarged front view, showing a detail of a welding head inthe working configuration of FIG. 9;

FIG. 11 is an enlarged plan bottom view, showing two gripping members ofa welding head in the working configuration of FIG. 9;

FIG. 12 is an enlarged plan bottom view, showing the gripping members ofFIG. 11 in the working configuration of FIG. 6.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a lid 1 intended to be applied onto a container neck. Thelid 1 comprises an end wall 2, which may be shaped as a disk,particularly a circular disk. The lid 1 further comprises a side wall 3,which may be substantially cylindrical, appended to the end wall 2.

The side wall 3 extends around an axis of the lid 1, whereas the endwall 2 extends transversely, in particular perpendicularly, to the axisof the lid 1.

The side wall 3 is provided with one or more internal threads 4 capableof engaging with corresponding threads obtained on the container neck.The side wall 3 may be externally provided with a plurality of knurls 5,which allow a user to more easily grip the lid 1.

Two appendages 6 project from an outer surface of the side wall 3, atdiametrically opposite positions.

A welding promoting element 7, which may be for example disk-shaped, isanchored to an inner surface of the end wall 2, i.e. to a surface that,in use, faces the container neck.

The lid 1 may comprise an annular rib 8, obtained integrally with theend wall 2, which axially protrudes from the end wall 2 towards theinside of the lid 1. The annular rib 8 defines a seat for receiving thewelding promoting element 7.

The welding promoting element 7 is defined by a multilayer sheet elementdistinct from the end wall 2 and permanently connected to the latter. Inparticular, the welding promoting element 7 comprises a layer ofconductive material, e.g. an aluminium foil, and at least two layers ofheat-sealable plastic material, e.g. polyethylene films, covering bothsides of the layer of conductive material and defining respectiveopposite faces of the welding promoting element 7. A face of the weldingpromoting element 7 is welded to the end wall 2 by the heat generatedwhen inducing a current in the layer made of conductive material.

As shown in FIG. 2, the lid 1 is intended to be applied onto a neck 9 ofa container 10, particularly a sealed container for packaging pourablefood products.

The neck 9 is included in an opening device comprising a pouring spout11 fixed to the packaging material forming the container 10. The neck 9has a substantially cylindrical tubular shape and extends around an axisA. The neck 9 defines a pouring opening 12, through which the content ofthe container 10 can be poured by a user. On an outer surface of theneck 9, extending around the axis A, one or more outer threads 13 areprovided, which are capable of engaging with the inner threads 4 of thelid 1. Thus, the lid 1 can be removably coupled to the neck 9. When thelid 1 is screwed onto the neck 9, the axis of the lid 1 is coincidentwith the axis A of the neck 9.

The pouring opening 12 is initially closed by a closing element 14 thatis integrally connected to the pouring spout 11 at a tear line 15, alongwhich the closing element 14 can be detached from the pouring spout 11.

The pouring spout 11 and the closing element 14 are formed in one pieceon a receiving portion 16 of the packaging material forming thecontainer 10, whilst the lid 1 is formed separately from the pouringspout 11 and the closing element 14, and then fitted thereto. Thepouring spout 11 and the closing element 14 may be obtained by mouldingmolten plastic material—in particular by an injection mouldingoperation—on the packaging material before it is transformed into acontainer 10.

The receiving portion 16 may be defined by a so called pre-laminatedhole made in the packaging material forming the container 10, i.e. ahole made through a base layer of the packaging material and covered byone, two, or more lamination layers of the packaging material, whichseal the hole. In an alternative embodiment, the receiving portion 16may be simply defined by a hole made through the whole thickness of thepackaging material forming the container 1, which is intended to besealed by the pouring spout 11 and the closing element 14. In anotherembodiment, the receiving portion 16 may be defined by a patch fixed tothe rest of packaging material to seal a hole formed, in this case,through the whole thickness of the packaging material.

The closing element 14 is formed in one piece with a protruding portion17 extending through the pouring opening 12. The protruding portion 17is intended to be welded to the welding promoting element 7, after thelid 1 has been screwed onto the neck 9.

The protruding portion 17 may comprise an annular body for contactingthe welding promoting element 7, and two or more legs 19 for connectingthe annular body 18 to the closing element 14. In the example shown, thelegs 19 are diametrically opposite one another.

The container 10 is formed, filled and sealed in a filling machine andis already provided with the pouring spout 11, the closing element 14and the protruding portion 17 when leaving the filling machine.Thereafter, as will be described in greater detail hereinbelow, anapplying head screws a lid 1 onto the neck 9. The welding promotingelement 7 of the lid 1 is then welded to the protruding portion 17, byinducing an electric current in the conductive layer of the weldingpromoting element 7. This current melts the heat-sealable layer of thewelding promoting element 7 that faces the neck 9, thereby permanentlyjoining the welding promoting element 7 to the protruding portion 17.

In use, the first opening of the container 10 is obtained by rotatingthe lid 1 relative to the pouring spout 11 around the axis A. At thebeginning of the rotation impressed by the user on the lid 1, the legs19 bend in the direction of rotation, thereby exerting a pulling actionon the closing element 14 at a given point of the tear line 15. In otherwords, due to the legs 19, the torque exerted on the lid 1 istransformed in a pulling action on the closing element 14, which startsto detach from the pouring spout 11 at two given points along the tearline 15.

By continuing to rotate the lid 1, the latter unscrews completely fromthe pouring spout 11. As shown in FIG. 3, the closing element 14 isfully detached from the pouring spout 11 along the tear line 15, andremains attached to the lid 1. At this point, the user can pour thecontent of the container 10 through the pouring opening 12.

The user can then use the lid 1 to again close or open the container 10,by screwing the lid 1 onto the neck 9 or respectively unscrewing the lid1 from the neck 9, as often as desired.

FIG. 4 shows an applying unit or apparatus 20 for applying lids 1 ontorespective containers 10, which have already been formed, filled andsealed in a filling machine provided upstream of the apparatus 20.

Each container 10 has, in the embodiment shown, a substantiallyparallelepiped body and a slanted top wall, i.e. a top wall that isinclined with respect to a base wall of the container 10. In otherwords, when the base wall of the container 10 rests on a horizontalsurface, the top wall is inclined with respect to a horizontal planeparallel to the base wall.

In another embodiment, not shown, each container 10 has a substantiallyparallelepiped body and a top wall substantially parallel to the basewall.

In general, the apparatus 20 may work with containers 10 havingdifferent shapes and/or dimensions.

The apparatus 20 comprises a conveying device, which is not shown, foradvancing the containers 10 in an advancement direction F, particularlyalong a rectilinear path. The conveying device may comprise a linearconveyor, for example a belt conveyor or a roller conveyor. Theconveying device may be configured to advance the containers 10continuously along the advancement direction F.

Upstream of the conveying device, a sequencing device may be provided,in order to arrange the containers 10 at a preset distance one fromanother, i.e. in a sequence having a preset pitch. It is thus ensuredthat the containers 10 are regularly spaced when entering the apparatus20.

The apparatus 20 further comprises a distribution unit for feeding thelids 1 to the necks 9 of the containers 10 advancing along theadvancement direction F. The distribution unit 21 may comprise a slide22 and a feeding unit, which is not shown, for supplying the lids 1 tothe slide 22. The slide 22 is inclined with respect to the conveyingdevice that conveys the containers 10 along the advancement direction F,so that the distance between the slide 22 and the conveying devicedecreases when moving along the advancement direction F.

At a lower end of the slide 22, a dispensing opening is provided, whichis so configured as to loosely retain one lid 1.

When a container 10, advanced along the advancement direction F, reachesthe slide 22, the neck 9 of the container 10 interacts with the lid 1retained by the dispensing opening of the distribution unit 20 andremoves the lid 1 from such dispensing unit. In other words, the lid 1is stripped from the dispensing opening located downstream of the slide22 by the neck 9 of an underlying container 10. Therefore, downstream ofthe slide 22, the lid 1 is positioned on the neck 9, although not beingfully screwed onto the neck 9. The apparatus 20 further comprises agroup of applying heads 23 for applying the lids 1 to the respectivecontainers 10, particularly by screwing each lid 1 onto a correspondingneck 9 of a container 10.

In the embodiment shown, the group of applying heads comprises twoapplying heads 23, arranged in sequence along the advancement directionF. However, the group of applying heads may comprise also a number ofapplying heads 23 different from two, for example three, four or moreapplying heads 23. Each applying head 23 is arranged to screw a lid 1onto a corresponding neck 9.

The applying heads 23 are supported by a common support element 24. Thesupport element 24 may be formed in one piece, or may be assembled froma plurality of components that are connected one to another so as tobehave, in use, like a single piece.

In the example shown, the support element 24 is shaped as a back platefrom which a plurality of projecting walls extend, so as to define aplurality of recesses 25, in each of which an applying head 23 ispartially housed. The support element 24 may nevertheless have shapesdifferent from that shown in the drawings.

The applying heads 23 are fitted to the support element 24 so that thedistance between two consecutive applying heads 23 is equal to thedistance between two consecutive containers 10 advanced along theadvancement direction F, i.e. to the pitch of the containers 10. Amovement device 26 is provided for moving the group of applying heads 23along the advancement direction F, so that each applying head 23 iscoupled to an underlying container 10 for a portion of the path thereofalong the advancement direction F. In other words, the movement device26 allows the applying heads 23 to follow the containers 10 with whichthey interact along a portion of the path of the containers 10. Theapplying heads 23 are hence movable synchronously with the conveyingdevice that advances the containers 10 in the advancement direction F,along the above mentioned portion of the path of the conveying device.

The movement device 26 may comprise a motor 27, possibly connected to agear box 28, arranged for driving a pulley 30 around which a belt 29 iswound. The belt 29 is further wound around a driven pulley 31. The belt29 has an active branch that is intended to drive the support element 24and runs parallelly to the advancement direction F.

A pair of guiding bars 32, each of which extends parallelly to theadvancement direction F, is provided for guiding the support element 24as the latter moves along the advancement direction F. In particular, asliding element that is not shown is slidable along the guiding bars 32.The support element 24 is fixed relative to the sliding element.

One or more connecting plates, which are not shown, are fastened to thebelt 29. The connecting plates are fixed relative to the sliding elementmentioned above.

The motor 27 is configured for rotating the pulley 30 in two oppositerotation directions, so that the belt 29 may be driven forwards orbackwards parallelly to the advancement direction F.

When the active branch of the belt 29 moves parallelly to theadvancement direction F, the support element 24—which, as explainedabove, is fixed relative to the belt 29—moves together with the belt 29.The applying heads 23, which are supported by the support element 24,are therefore moved forwards or backwards along the advancementdirection F. In particular, when the applying heads 23 are movedforwards along the advancement direction F, the applying heads 23 canfollow respective containers 10 so that each applying head 23 applies alid 1 on the corresponding neck 9. After disengaging from respectivecontainers 10, the applying heads 23 can be moved backwards along theadvancement direction F so as to be brought back to a starting point atwhich the applying heads 23 can start to interact with new containers10.

The movement device 26 thus moves synchronously the applying heads 23,which means that the applying heads 23 are moved together along theadvancement direction F by the movement device 26. In other words, theapplying heads 23 are moved by the movement device 26 simultaneously andby the same extent, along the advancement direction F.

In an embodiment that is not shown, the movement device may comprise atoothed rack fixed relative to the support element 24. The motor 27 hasa shaft coupled to a toothed wheel or pinion that engages with thetoothed rack, so as to move the latter forwards or backwards parallellyto the advancement direction F.

More in general, the movement device 26 may comprise any transmissiondevice that is capable of transforming rotation of a shaft of the motor27 into a linear motion of a linearly movable element fixed relative tothe support element 24.

As shown in FIGS. 5 to 8, each applying head 23 comprises a pair ofgripping elements 33 intended to interact with the lid 1 for screwing itonto the neck 9. The gripping elements 33 of an applying head 23 aresupported by a supporting component 35 connected to a stem 36. Eachapplying head 23 further comprises a driving device 34 for driving thegripping elements 33. The driving device is fastened to the supportelement 24. The driving device 34 is directly connected to the stem 36,so as to move the supporting component 35 and hence drive simultaneouslyboth the gripping elements 33.

In particular, the driving device 34 is capable of causing the stem 36to slide parallelly to the longitudinal axis thereof, thereby displacingthe supporting component 35 and hence the gripping elements 33 in adirection parallel to the axis A of a neck 9. The gripping elements 33can thus be moved towards a lid 1 to be screwed, or alternatively bemoved away from a lid 1 that has already been screwed onto thecorresponding neck 9. To this end, the driving device 34 may comprise alinear motor.

The driving device 34 is furthermore configured to rotate the stem 36,and hence the supporting component 35 and the gripping elements 33,around a longitudinal axis of the stem 36 that, in use, coincides withthe axis A of a neck 9.

By rotating the supporting component 35 around the longitudinal axis ofthe stem 36, while the latter is moved towards the container 1, thegripping elements 33 move along a helix, thereby screwing the lid 1 ontothe neck 9, as will be described in greater detail hereinbelow.

As shown in FIGS. 7 and 8, each gripping element 33 comprises anabutment plane 38 for contacting the end wall 2 of the lid 1. Theabutment plane 38 is, for example, obtained on a main portion 39 of thegripping element 33, which may be approximately semicircular in planview.

Each gripping element 33 further comprises a screwing portion 40, whichprojects from the abutment plane 38 towards the container 10, i.e. awayfrom the supporting component 35. The screwing portion 40 is delimitedby two active surfaces 41, each of which is capable of interacting withan appendage 6 of a lid 1. In the example shown, the active surfaces 41are substantially flat and each active surface 41 lies in a plane thatis parallel to the axis A of the neck 9.

The screwing portion 40 is further delimited by two joining surfaces 42,each of which is interposed between the active surfaces 41. The joiningsurfaces 42 may be shaped as portions of cylindrical surfaces,particularly coaxial one to another.

Each gripping element 33 comprises a guiding portion 37, for exampleT-shaped, which is slidable inside a correspondingly shaped seatobtained in the supporting component 35. The guiding portion 37 projectsfrom a face of the main portion 39 opposite a further face of the mainportion 39 from which the screwing portion 40 projects.

The gripping elements 33 of an applying head 23 are movable between anengagement position P1, shown in FIGS. 6 and 8, and a disengagementposition P2, shown in FIGS. 5 and 7. In the engagement position P1, thegripping elements 33 are close to one another, so that the activesurfaces 41 of the screwing portions 40 may interact with the appendages6 of a lid 1 in order to rotate the lid 1 around the axis A of the neck9. In the disengagement position P2, the gripping elements 33 are farfrom one another, so that the screwing portions 40 are arrangedexternally of the appendages 6 and cannot interact with the appendages6.

When moving from the disengagement position P2 to the engagementposition P1 or vice versa, the gripping elements 33 move towards oneanother or away from one another along a direction that is perpendicularto the axis A of the neck 9. The guiding portions 37 slide in the seatsof the supporting component 35, which keep the gripping elements 33guided along their stroke from the engagement position P1 to thedisengagement position P2 or vice versa.

A pneumatic arrangement is provided for displacing the gripping elements33 from the engagement position P1 to the disengagement position P2 orvice versa. The pneumatic arrangement comprises a pneumatic circuitinside which a pneumatic fluid, e.g. compressed air, may flow. Thepneumatic circuit comprises a conduit defined inside the stem 36, whichis hollow, such conduit being in fluid communication with a chamberobtained inside the supporting component 35. From this chamber, thepneumatic fluid acts on the guiding portions 37 of the gripping elements33 in order to displace the gripping elements 33 between the engagementposition P1 and the disengagement position P2.

In use, the driving device 34 moves the gripping elements 33 along adirection parallel to the axis A of a neck 9, towards the lid 1, untilthe abutment plane 38 abuts against the end wall 2 of the lid 1, asshown in FIG. 7. From this position, the gripping elements 33 movetowards the lid 1 slightly more, so as to exert on the lid 1 a forcedirected towards the container 10, i.e. a compressing force, whichallows the lid 1 to be positioned correctly on the neck 9. The grippingelements 33 thus exert a leveling action on the lid 1, in order toensure that the end wall 2 of the lid 1 is perpendicular to the axis Aof the neck 9, i.e. that the lid 1 is arranged coaxially to the neck 9.

This helps in applying correctly the lid 1 onto the neck 9, which mightbe a critical operation, as explained below.

It may happen that, when the lid 1 is stripped by a container 10 in thedistribution unit 21, the lid 1 is not correctly positioned onto theneck 9. For example, the lid 1 might rest on the neck 9 in a tiltedposition, in which the axis A of the neck 9 is inclined relative to anaxis of the neck 9.

This situation may be worsened because the containers 10 are not rigidcontainers, but are made of a flexible packaging sheet material and havedimensions varying within relatively large tolerances between differentcontainers. Furthermore, the stiffness of the containers is not as highas that of a glass bottle or jar.

Consequently, the containers 10 need to be handled with care, to ensurethat they are not damaged by the apparatus 20.

In addition, the neck 9 may be relatively soft, particularly if mouldeddirectly on the sheet packaging material. For this reason, the neck 9might be slightly elliptical, instead of being perfectly circular.

In this situation, the compressing force applied by the grippingelements 33 on the lid 1 before starting to screw the lid 1 on the neck9, may adapt the lid 1 to the neck 9, thereby enabling the lid 1 to besubsequently more easily screwed on the neck 9.

It is stressed that the leveling action exerted on the lid 1 beforestarting to screw the latter is carried out under controlled conditions,so that the neck 9 is not damaged. To this end, using a linear motorincluded in the driving device 34 is particularly helpful, because alinear motor allows the applied forces to be easily controlled.

While the abutment plane 38 applies on the lid 1 the compression forcethat allows the lid 1 to be leveled on the neck 9, the gripping elements33 are still in the disengagement position P2. After leveling the lid 1,the gripping elements 33 are moved one towards another by the pneumaticarrangement associated thereto, so that they can reach the engagementposition P1.

It is stressed that, in the engagement position P1, the distance betweenthe gripping elements 33 is not sufficiently short to allow the screwingportion 40 to interact with the side wall 3 of the neck 9, because thejoining surfaces 42 of the screwing portion 40 are not in contact withthe side wall 3. The lid 1 is screwed by the active surfaces 41 of thescrewing portions 40, which act on the appendages 6.

Hence, after the gripping elements 33 are moved close to one another soas to reach the engagement position P1, the gripping elements 33 mightneed to be rotated by a certain angle around the axis of the stem 36, soas to bring an active surface 41 of each screwing portion 40 in contactwith an appendage 6 of the lid 1, as shown in FIG. 8. At this point, thescrewing portions 40 are actually engaged with the appendages 6 and mayeffectively rotate the lid 1.

Since, in order to rotate the lid 1, the gripping elements 33 engagewith the appendages 6 that protrude from the side wall 3, possiblemisalignments between the lid 1 and the stem 36 can be compensated.

In particular, the gripping elements 33 may initially rotate the lid 1in a rotation direction that is opposite the rotation direction thatallows the lid 1 to be screwed onto the neck 9, i.e. in an unscrewingdirection, which is usually counterclockwise. This allows the lid 1 tobe so positioned, that the starting point of an internal thread 4 on thelid 1 is aligned with the starting point of an outer thread 13 on theneck 9. This makes easier screwing the lid 1 onto the neck 9.

The gripping elements 33 are now rotated in a screwing direction, andsimultaneously moved towards the container 10, so that the lid 1 isscrewed onto the neck 9.

In the embodiment shown, in which the lid 1 comprises the appendages 6,the lid 1 is rotated in the screwing direction until the appendages 6have reached a preset position, particularly a preset angular positionrelative to the neck 9. At this point, the lid 1 is correctly screwedonto the neck 9 and can be released by the gripping elements 33. Inother words, a position control that controls the position of theappendages 6 is used for determining the end of the screwing operationallowing the lid 1 to be screwed onto the neck 9.

This position control is made possible by a position sensor,particularly an encoder, which is associated to the driving device 34.The sensor allows the angular position of the stem 36, and hence of thegripping elements 33, to be determined.

In an embodiment that is not shown, the applying heads 23 may beconfigured to apply, onto the containers 10, lids 1 that do not comprisethe appendages 6. i.e. lids 1 that are delimited by a cylindrical sidewall 3.

In this case, in order to screw a lid 1 onto the respective neck 9, thegripping elements 33 act on the side wall 3 of the lid 1, i.e. gripdirectly a portion of cylindrical surface delimiting the lid 1 aroundits axis. The screwing action terminates when a preset screwing forcehas been applied onto the lid 1. In other words, instead of determiningwhen the screwing operation is finished by controlling the position ofthe lid 1, the end of the screwing operation is determined bycontrolling the screwing force applied on the lid 1. This can be done bycontrolling the torque applied by the gripping elements 33 on the lid 1.It is thus possible to avoid damages on the lid 1 and/or on the neck 9due to an excessive screwing force.

Owing to the pneumatic arrangement, which moves the gripping elements 3between the engagement position P1 and the disengagement position P2,the force applied onto the lid 1 while rotating the latter can becontrolled easily and precisely, by controlling the pressure acting onthe guiding portions 37 of the gripping elements 33.

The risks of damaging the lids 1 and/or the necks 9 are thereforestrongly reduced.

The apparatus 20 further comprises a plurality of welding heads 43 forinduction welding the lid 1 to the protruding portion 17 that projectsfrom the closing element 14. In particular, each welding head 43 isconfigured to generate in the conductive layer of the welding promotingelement 7 an electric current, which melts the heat-sealable layer ofthe welding promoting element 7 facing the closing element 14. Thisheat-sealable layer thus adheres to the protruding portion 17, whichconsequently becomes permanently attached to the lid 1.

The welding heads 43 are arranged downstream of the applying heads 23,along the conveying device that advances the containers 10 in theadvancement direction F. The welding heads 43 thus interact with thecontainers 10 after the applying heads 23 have screwed the lids 1 ontothe containers 10.

The number of welding heads 43 may be equal to the number of applyingheads 23. In the example shown, two welding heads 43 are provided, butthis condition is not essential and the number of welding heads 43 couldalso be different from two.

The distance between two consecutive welding heads 43 is equal to thedistance between two consecutive containers advancing along theadvancement direction F.

Similarly, the distance between the last applying head and the firstwelding head 43 is equal to the distance between two consecutivecontainers 10.

The welding heads 43 are movable forwards and backwards in theadvancement direction F, i.e. are movable synchronously with theconveying device that advances the containers 10 in the advancementdirection F, along a part of the path of the conveying device. Thus,each welding head 43 is capable of following a container 10 in order tointeract with the corresponding lid 1 for a time sufficient to ensurewelding of the lid 1 to the protruding portion 17. When the lid 1 hasbeen welded to the protruding portion 17, the welding head 43 movesbackwards so as to reach an initial position in which it is ready toweld a new lid 1 to the corresponding container 10.

All the welding heads 43 can be moved synchronously by a commonmechanism. To this end, all the welding heads 43 can be supported by acommon support member 44 so that, by moving the support member 44 in theadvancement direction F, all the welding heads 43 are moved together inthe advancing direction F forwards or backwards.

In the example shown, the welding heads 43 are moved by the samemovement device 26 that also displaces the applying heads 23. Thesupport member 44 is fixed relative to the support element 24, forexample because the support member 44 is directly fastened to thesupport element 24. Thus, when the motor 27 moves the support element 24by driving the belt 29, the support member 44, and consequently also thewelding heads 43, are also displaced in the advancement direction F.

By using the same movement device 26 for moving both the applying heads23 and the welding heads 43, it is possible to obtain an apparatus 20that is structurally simple, since it comprises only one motor 27 toactuate a plurality of heads. The apparatus 20 is also easy to beoperated, since all the applying heads 23 and the welding heads 43 canbe simultaneously controlled as far as displacement along theadvancement direction F is concerned.

Furthermore, the position of the applying heads 23, of the welding heads43 and of the distribution unit 21 can be easily adjusted if, forexample, there is the need to change the size and/or shape of thecontainers 10 to be processed.

FIG. 4 shows a frame 45 of the apparatus 20, which is fixed relative tothe packaging line that processes the containers 10. The frame 45comprises two end brackets 46, arranged at opposite sides of theapparatus 20. The distribution unit 21, the applying heads 23 and thewelding heads 43 are supported by a common supporting arrangement thatcomprises, in addition to the support element 24 and the support member44, a structure supporting the movement device 26 and a structuresupporting the distribution unit 21. The common supporting arrangementcomprises two supporting plates 49 each of which is fastened to an endbracket 46. If there is a change in the shape of the containers 10 to beprocessed, for example because the inclination of the top wall of thecontainers 10 has been changed, it is sufficient to oscillate the commonsupporting arrangement that supports the distribution unit 21, theapplying heads 23 and the welding heads 43, relative to the frame 45.Said common supporting arrangement can in particular be oscillatedaround a fulcrum 47. A plurality of arcuate slots 48 are provided onboth the end brackets 46 in order to allow the supporting plates 49 tobe rotated relative to the end brackets 46.

If there is the need to change the height of the containers 10 to beprocessed, the vertical position of the distribution unit 21 can beeasily adjusted by vertically displacing the whole structure supportingthe distribution unit 21, which is made possible by adjusting slots 50provided in the supporting plate 49 adjacent to the distribution unit21. The height of the distribution unit 21 above the conveying devicethat advances the containers 10 in the advancement direction F cantherefore be adjusted.

Similarly, the vertical position of the applying module comprising theapplying heads 23 and of the welding module comprising the welding heads43 can be easily adjusted by means of few and simple adjustmentoperations. The support element 24 and the support member 44 areactually positioned in the supporting arrangement so that their heightrelative to the conveying device is adjustable.

Thus, the apparatus 20 has high flexibility.

FIG. 10 shows in detail a welding head 43 acting on a lid 1 of acontainer 10. The welding head 43 comprises an electric inductiongenerating element 51 including a coil that is not shown. By activatingthe coil, an electric current is induced in the conductive layer of thewelding promoting element 7, with a consequent generation of localizedheat causing the heat-sealable layer of the welding promoting element 7facing the neck 9 to be welded to the protruding portion 17.

The welding head 43 comprises an actuator device 52 for displacing theelectric induction generating element 51 in a displacement directionarranged transversely, in particular perpendicularly, to the advancementdirection F, so as to move the electric induction generating element 51towards a lid 1 or alternatively away from a lid 1. More precisely, theactuator device 52 is intended to move the electric induction generatingelement 51 parallelly to the axis of the lid 1 that is being welded.

The actuator device 52 may comprise for example an electric linear motoror a pneumatic actuator. The actuator device 52 is configured todisplace the electric induction generating element 51 between an activeconfiguration C1, shown for example in FIG. 10, and an inactiveconfiguration C2, shown in FIG. 4. In the inactive configuration C2, theelectric induction generating element 51 is at a distance from the lid1, so as not to interact with the latter. In the active configurationC1, the electric induction generating element 51 interacts with the lid1 and welds the latter to the protruding portion 17. In the embodimentshown, the electric induction generating element 51 is in contact withthe end wall 2 of the lid 1 in the active configuration C1. In anembodiment that is not shown, the electric induction generation element51 may nevertheless weld the lid 1 to the protruding portion 17 byremaining at a short distance from the end wall 2 in the activeconfiguration C1.

Each welding head 43 further comprises a pair of gripping members 53 forgripping the lid 1, particularly at the side wall 3 thereof, so as toavoid undesired side displacements of the lid 1 during welding.

As shown in FIGS. 11 and 12, each gripping member 53 comprises aninteraction portion 54 for interacting with the lid 1. The interactionportion 54 of a gripping member 53 is delimited by two contact surfaces55 for contacting the side wall 3 of the lid 1. The contact surfaces 55are substantially flat. In the example shown, the contact surfaces 55 ofa gripping member 53 define respective planes that are perpendicular toone another.

The gripping members 53 of a welding head 43 are symmetrical withrespect to one another. Hence, in the example shown, the four contactsurfaces 55 of the two interacting gripping members 53 lie in respectiveplanes that are arranged at right angles one to another, therebydefining an ideal square, in plan view.

In use, each contact surfaces 55 of a pair of gripping members 53 liesin a respective plane parallel to the axis of a lid 1.

Between the two contact surfaces 55 of a gripping member 53, a recess 56is provided in the interacting portion 54. The recess 56 has the purposeof housing an appendage 6 of a lid 1 while the lid 1 is gripped by thegripping members 53.

The gripping members 53 of a welding head 43 are movable between an openposition Q1, shown in FIG. 11, and a closed position Q2, shown in FIG.12. In the open position Q1, the gripping members 53 are far away fromone another and do not interact with a lid 1. In the closed position Q2,the gripping members 53 are closer to one another, so as to hold a lid 1between the contact surfaces 55.

As shown in FIGS. 11 and 12, each welding head 43 further comprises acarrying element 57 for supporting the gripping members 53. The carryingelement 57 may be arranged in a back region of the welding head 43.

Each gripping member 53 comprises a guiding part 58 that is slidinglycoupled to the carrying element 57. The guiding part 58 may be forexample T-shaped and engages in a correspondingly shaped seat of thecarrying element 57, so as to be guided by the carrying element 57 whenthe gripping members 53 move between the open position Q1 and the closedposition Q2.

Each gripping member 53 further comprises an intermediate portion 59that connects the interaction portion 54 to the guiding part 58.

A pneumatic system is provided for displacing the gripping members 53between the open position Q1 and the closed position Q2. The pneumaticsystem comprises a pneumatic circuit inside which a pneumatic fluid,e.g. compressed air, may flow. The pneumatic circuit comprises a chamberobtained inside the carrying element 57. From this chamber, thepneumatic fluid acts on the guiding parts 58 of the gripping members 53in order to displace the gripping members 53 between the open positionQ1 and the closed position Q2.

The carrying element 57 is fastened to a mounting component 60 arrangedabove the electric induction generating element 51.

Each welding head 43 further comprises a compensating device 62 thatallows the electric inducting generating element 51 to be centeredrelative to the end wall 2 of the lid 1, by compensating a possiblemisalignment between the electric inducting generating element 51 andthe lid 1. The compensating device 62 is interposed between the actuatordevice 52 and the electric inductive generating element 51.

The compensating device 62 comprises a first component 63 that is fixedrelative to the actuator device 52. In particular, the first component63 is fixed relative to a plate 64 of the actuator device 52. The plate64 is in turn connected to a plurality of columns 65 of the actuatordevice 52, the columns 65 being slidable relative to a main body 66 ofthe actuator device 52 when the actuator device 52 moves the grippingmembers 53 between the active configuration C1 and the inactiveconfiguration C2.

The compensating device 62 further comprises a second component 67coupled to the first component 63. In particular, the coupling betweenthe first component 63 and the second component 67 has two degrees offreedom, since the second component 67 is capable of sliding in twodirections relative to the first component 63. More precisely, the firstcomponent 63 and the second component 67 are in contact to one anotherin an interface plane P and the second component 67 is capable ofsliding in the interface plane P relative to the first component 63. Theinterface plane P is parallel to a flat surface of the electricinduction generating element 51, said flat surface being intended tocontact the end wall 2 of the lid 1.

The interface plane P is arranged transversely, in particularperpendicularly, to the displacement direction along which the actuatordevice 52 displaces the electric induction generating element 51 whenmoving the latter between the active configuration C1 and the inactiveconfiguration C2.

In use, the interface plane P is arranged transversely, in particularperpendicularly, to the axis A of the neck 9. In other words, theinterface plane P is parallel to the end wall 2 of the lid 1 and also toa top wall of the container 10.

The second component 67 is closer to the electric induction generatingelement 51 than the first component 63. In other words, the secondcomponent 67 faces towards the electric induction generating element 51,whereas the first component 63 faces away from the electric inductiongenerating element 51.

The mechanism that allows second component 67 to slide relative to thefirst component 63 will not be described in detail, since thecompensating device 62 is known per se.

The electric induction generating element 51 is fixed relative to thesecond component 67 as far as displacement in a plane parallel to theinterface plane P is concerned.

As will be described in greater detail hereinbelow, when the weldinghead 43 interacts with a lid 1 in order to weld the latter to aprotruding portion 17, the electric induction generating element 51 isfirst brought in the active configuration C1. The gripping members 53are still in the open position Q1, as shown in FIG. 5. The grippingmembers 53 are then brought into the closed position Q2 by the pneumaticsystem associated thereto. It may happen that the cap 1 is not perfectlycentered relative to the electric induction generating element 51 andhence relative to the gripping members 53. If this occurs, a center ofthe end wall 2 of the lid 1 is not coincident with a central point of anideal quadrilateral defined by the contact surfaces 55. Hence, when thegripping members 53 move one towards another to reach the closedposition Q2, the side wall 3 of the lid contacts one of the contactsurfaces 55 before contacting the other three contact surfaces 55. Underthe contact force exerted by the lid 1 on the first contact surface 55with which the lid 1 interacts, the second component 67 of thecompensating device 62 moves parallelly to the interface plane P, i.e.parallelly to the end wall 2 of the lid 1, in order to compensate suchcontact force. The electric induction generating element 51 movestogether with the second component 67, thereby reaching a position inwhich the gripping members 53, and hence also the electric inductiongenerating element 51, are centered relative to the lid 1.

In other words, the gripping members 53 exert a guiding action on thesecond component 67 of the compensating device 62, and consequently onthe electric induction generating element 51. In particular, by means ofthe contact surface 55 that firstly contacts the lid 1, the lid 1determines the position of the gripping members 53, which in turndetermine the position of the second component 67, thereby allowing theelectric induction generating element 51 to be centered relative to thelid 1. This is made possible because the gripping members 53 areassociated with the second component 67, particularly by means of thecarrying element 58, which is fixed relative to the second component 67as far as displacement parallelly to the interface plane P is concerned.

At this point, the electric induction generating element 51 maycorrectly weld the end wall 2 of the lid 1 to the protruding portion 17.

In operation, the containers 10 are arranged in a sequence at a presetdistance from one another by a sequencing device that is not shown. Thecontainers 10 are then advanced along the advancement direction F so asto pass below the distribution unit 21. Here, each container 10 receivesa lid 1, which rests on the neck 9 without being fully applied to thelatter. The lid 1 may be released in a random angular position on theneck 9, i.e. the distribution unit 21 may be so configured that aposition control on the angular position of the lids 1 is not applied.

Subsequently, the containers 10 arrive below the applying heads 23, andthe gripping elements 33 of each applying head 23 are verticallydisplaced by the respective driving device 34, so as to bring theabutment plane 38 of each gripping element 33 in contact with the endwall 2 of each lid 1. The gripping elements of each gripping head 23,which are still in the disengagement position P2, as shown in FIG. 5,apply a compressing force on the respective lid 1, i.e. push the lid 1towards the neck 9. This allows the lid 1 to be leveled on the neck 9,i.e. to be positioned on the neck 9 with the top wall 2 substantiallyperpendicular to the axis of the neck 9. The gripping elements 33 ofeach applying head 23 are then moved into the engagement position P1, asshown in FIG. 6, and the lid 1 is initially rotated in an unscrewingdirection so that its internal thread 4 engages with the outer thread 13of the neck 9. Thereafter, the lid 1 is rotated in a screwing direction,so as to be screwed onto the neck 9, until a predetermined angularposition of the lid 1, or a predetermined screwing force, has beenreached.

The operations described above, aimed at screwing a lid onto acorresponding neck 9, are carried out simultaneously on respective lids1 by all the applying heads 23 of the group of applying heads 23, i.e.by all the applying heads 23 supported by the support element 24. In themeantime, the support element 24 is moved in the advancement direction Fby the movement device 26, so that the applying heads 23 advance alongthe advancement direction F together with the containers 10, whileapplying thereon the respective lids 1.

When the lids 1 have been completely screwed onto the necks 9, thegripping elements 33 of each applying head are opened to reach thedisengagement position P2, and are displaced upwards in order todisengage from the corresponding lid 1, as shown in FIG. 9. The movementdevice 26 now displaces the applying heads 23 backwards, so that theapplying heads 23 can engage with a new group of containers 10.

The containers 10 on which the lids 1 have been applied continue to movealong the advancement direction F towards the welding heads 43, so as toarrive below the welding heads 43. Each welding head 43 is thendisplaced downwards, until the respective electric induction generatingelement 31 contacts the end wall 2 of a lid 1 in the activeconfiguration C1. The gripping members 53 are in the open position Q1,as shown in FIGS. 5 and 9.

After reaching the active configuration C1, the gripping members 53 ofeach welding head 43 are brought into the closed position Q2, as shownin FIG. 6. The side wall 3 of the lid 1 is contacted by at least onecontact surface 55 of a gripping member 53. Depending on which contactsurface 55 contacts the side wall 3 of the lid 1, the compensatingdevice 62 allows the electric induction generating element 51 to move ina plane perpendicular to the end wall 2 of the lid 1, so as to reach acentered position relative to the lid 1. The lid 1 may now be welded tothe protruding portion 17 of the closing element 14, owing to thewelding promoting element 7.

While the welding heads 43 interact with the containers 10, the lattercontinue to be advanced along the advancement direction F. The weldingheads 43 thus weld the lids 1 on the respective containers 10 whilemoving together with the containers 10 that are advanced along theadvancement direction F. To this end, the movement device 26 moves thesupport member 44 along the advancement direction F, which causes thewelding heads 43, which are fixed relative to the support member 44, tobe moved together, i.e. synchronously, along the advancement directionF.

Once the lids 1 have been welded onto the necks 9, the gripping members53 are opened and move away from the corresponding lid 1, which is thusreleased from the welding head 43. The welding heads 43 supported by thesupport member 44 are moved backwards by the movement device 26, so asto reach a position in which the welding heads 43 may engage with a newgroup of containers 10.

It is intended that, while the applying heads 23 interact with a groupof containers 10 by screwing thereon corresponding lids 1, the weldingheads 43 interact with a further group of containers 10 by welding thecorresponding lids 1 onto respective protruding portions 17. Thus, thescrewing and welding operations that were described above are carriedout simultaneously on different groups of containers 10.

In an alternative embodiment, the apparatus 20 may be used to apply,onto the necks 9, lids 1 that are substantially round, i.e. that do notpossess the appendages 6. In this case, the gripping elements 33 and thegripping members 53 may have a simpler structure and be configured toengage directly with the side wall 3 of the lids 1.

In any case, the apparatus 20, and particularly the welding heads 43thereof, allow the lids 1 to be reliably and effectively welded to theopening devices provided on the containers 10, and particularly to theprotruding portions 17, even at high production rates. Furthermore, thegripping elements 33 and the gripping members 53 can be shaped asdesired, thereby allowing also non-round lids 1 to be applied.

The invention claimed is:
 1. A welding head for welding a lid to acontainer having an opening device, the welding head comprising: awelding element for welding the lid to the opening device of thecontainer comprising at least two gripping members for holding the lidduring welding; and a compensating device configured to center thewelding element relative to the lid upon detection of a misalignmentbetween the welding element and the lid comprising: a first componentfacing away from the welding element; and a second component facingtowards the welding element, wherein the at least two gripping membersare configured to position the second component based on a position ofthe lid.
 2. The welding head according to claim 1, wherein thecompensating device is configured to move the welding element in a planeextending transversely to an axis of the lid.
 3. The welding headaccording to claim 1, wherein the first component interacts with thesecond component through an interface plane, and wherein the secondcomponent is configured to slide parallel to the interface planerelative to the first component.
 4. The welding head according to claim3, wherein the second component supports the welding element.
 5. Thewelding head according to claim 1, further comprising: a carryingelement that is fixed relative to the second component with respect toan interface plane, and wherein the carrying element supports thegripping members.
 6. The welding head according to claim 5, wherein eachgripping member further comprises: a guiding part that slidingly engagesin a corresponding seat of the carrying element, wherein the grippingmembers are configured to move between an open position, in which thegripping members do not interact with the lid, and a closed position, inwhich the gripping members hold the lid.
 7. The welding head accordingto claim 1, wherein the gripping members further comprise: a contactsurface for contacting at least a portion of a side wall of the lid andfor determining a position of the gripping members with respect to aninterface plane.
 8. The welding head according to claim 7, wherein eachgripping member comprises at least two contact surfaces for contactingrespective portions of the side wall of the lid and for determining arespective position of the respective gripping member with respect tothe interface plane.
 9. The welding head according to claim 8, whereineach gripping member comprises a recess located between the at least twocontact surfaces, wherein the recess houses an appendage protruding fromthe side wall of the lid.
 10. The welding head according to claim 8,wherein the contact surfaces of each gripping member define respectivecontact planes that are perpendicular to one another.
 11. The weldinghead according to claim 1, and further comprising: an actuator fordisplacing the welding element in a displacement direction and betweenan active configuration, in which the welding element is closer to anend wall of the lid for welding the end wall to the opening device, andan inactive configuration, in which the welding element is further awayfrom the end wall.
 12. The welding head according to claim 11, whereinthe actuator further comprises a moveable plate and the second componentis fixed relative to the movable plate.
 13. The welding head accordingto claim 12, wherein the displacement direction is transverse to aninterface plane.
 14. The welding head according to claim 1, wherein thewelding element further comprises: an electric induction generatingelement for generating an electric current configured to heat-seal thelid to the opening device.
 15. A welding head for welding a lid to acontainer having an opening device, wherein the lid has an axis, a sidewall parallel to the axis, and an end wall transverse to the axis, thewelding head comprising: a welding element for welding the lid to theopening device of the container; an actuator for displacing the weldingelement in a displacement direction parallel to the axis of the lid; anda compensating device configured to move the welding element into axialalignment with the lid, wherein the compensating device furthercomprises: at least two gripping members, each having a contact surfacefor contacting respective portions of the side wall of the lid and fordetermining a respective position of the respective gripping member; acoupling component coupled to the gripping members and configured tomove in an interface plane extending transverse to the lid axis, whereinmovement of the coupling component in the interface plane moves thewelding element transverse to the lid axis.